The injection molding of plastics generally involves introducing molten plastic under pressure into a space defined between a core part and a cavity part of an injection mold. The molten plastic is allowed to cool and thereby solidify to form a "part" in the space, after which the core and cavity parts are separated. The part generally shrinks a bit upon cooling and remains on the core part of the mold from where it must be "stripped".
Simple parts having no internal protrusions may be stripped in a variety of ways, including air pressure, stripper plates or rings and ejector rods. A stripper plate is basically a plate which lies against the core part of the mold during injection and through which individual cores extend. The stripper plate, which is moved away from the core part during stripping, presses against an innermost edge of the molded part to urge the part off of the core part of the mold.
Threaded parts are more difficult to strip. Parts with shallow threads may sometimes be forced off of a core using a stripper plate. Deeper threads however would be damaged by any effort to force them off the core with a stripper plate.
The optimal mechanism for removing a threaded part from a core is a simultaneous combination of pressure away from the core and rotation or "unthreading". Simple rotation on its own may damage the newly formed threads as parts are generally stripped while still warm to maximize output and therefore, the threads are still somewhat soft during stripping.
Mechanisms for removing threaded closures from injection molds are well known in the industry. One common method includes a rotating core to unscrew the molded closure from the core and eject the part. A major problem with this mechanism is one of water leakage due to the use of rotary seals necessary because the cores are typically water cooled. To overcome this problem, injection molds having stationary cores have been developed, for example, U.S. Pat. No. 5,383,780 (McCready et al.)
U.S. Pat. No. 5,383,780 (McCready et al.) and related U.S. Pat. Nos. 5,565,223 and 5,798,074 disclose apparatus for removing molded threaded parts using a movable, rotatable stripper ring. McCready utilizes what is referred to as a pinion to rotate a stripper ring coaxial with a mold core . The pinion is mounted for movement with the stripper ring. The pinion receives rotational input from a rack through a gear . The rack in turn receives input from a hydraulic cylinder mounted transversely to the molding machine axis.
In McCready, translational movement is provided by a pneumatic piston extending longitudinally relative to the machine axis and acting on the stripper ring support structure. Translational movement is controlled by a cam secured to and moveable with the rack. The cam does not move the stripper ring but instead prevents the degree of movement that would otherwise occur as a result of the pneumatic piston being actuated.
While McCready does provide an apparatus for synchronously, rotating and translating a stripper ring, it is mechanically quite complex requiring both hydraulic and pneumatic cylinders which must be coordinated with the molding machine cycle. This requires special machine logic which can only be incorporated in certain models of molding machines. Furthermore, hydraulic cylinders are prone to leakage which can contaminate the parts being formed and the hydraulic cylinder and its associated structure are quite bulky, requiring a considerable amount of space.
Pneumatic cylinders are also prone to leak which results in compressed air leakage. Compressed air leakage can cause contamination due to condensation. Contamination of any kind would prevent the use of a mold in a "clean room" environment. A clean room environment is often required in closure manufacturing.
Finally, the McCready arrangement is relatively expensive due to the complexity of the structures involved and the additional machine logics.
An object of the present invention is to provide an apparatus for rotating and translating a stripper ring which apparatus is driven by a machine ejector of an injection molding machine rather than a separate remote drive structure.
A further object of the present invention is to provide an apparatus for ejecting threaded injection molded parts which is suitable for a clean room environment without fear of contamination.
A still further object of the present invention is to provide an apparatus for ejecting threaded injection molded parts which may be used with a basic injection molding machine without adding special machine logic.
Yet another object of the present invention is to provide an apparatus useable with an unlimited number of cavitations, preferably in multiples of four, including a 72 cavity mold.